There are large spatial variations in agriculture and crop
rotation systems in China (Frolking et al., 2002; Qiu et al.,
2003). In this study we focused on 13 provincial-level
administrative units (12 provinces plus Shanghai) in southern
China (Fig. 1), which represent over 2.5 million km2 of
land area (Table 1). Two-crop rotation systems are dominant
across southern China, because of a long warm season and
abundant precipitation